1. Field
Aspects of embodiments according to the present invention relate to an organic light emitting display device.
2. Description of the Related Art
Recently, flat panel display devices having reduced weight and volume in comparison to a cathode ray tube are being developed. The flat panel display devices include liquid crystal displays, field emission displays, plasma display panels and organic light emitting display devices, and the like.
The organic light emitting display device displays an image using organic light emitting diodes that produce light by recombining electrons and holes. The organic light emitting display device has the advantage that it has fast response speed and is driven at low power.
FIG. 1 is a circuit diagram illustrating a pixel of an organic light emitting display device in the related art.
Referring to FIG. 1, a pixel 4 of an organic light emitting display device of the related art includes: an organic light emitting diode OLED; and a pixel circuit 2 coupled with a data line Dm and a scan line Sn for controlling the organic light emitting diode OLED.
The anode electrode of the organic light emitting diode OLED is coupled to the pixel circuit 2, and the cathode electrode is coupled to a second power supply ELVSS. The organic light emitting diode OLED produces light with predetermined luminance in response to the current supplied from the pixel circuit 2.
The pixel circuit 2 controls the amount of current supplied to the organic light emitting diode OLED, in response to a data signal supplied to the data line Dm, when a scan signal is supplied to the scan line Sn. For this configuration, the pixel circuit 2 includes: a second transistor M2 coupled between a first power supply ELVDD and the organic light emitting diode OLED; a first transistor M1 coupled to the second transistor M2, the data line Dm, and the scan line Sn; and a storage capacitor Cst coupled between a gate electrode and a first electrode of the second transistor M2.
A gate electrode of the first transistor M1 is coupled to the scan line Sn, and a first electrode of the first transistor M1 is coupled to the data line Dm. Further, a second electrode of the first transistor M1 is coupled to one terminal of the storage capacitor Cst. In this configuration, the first electrode is any one of a source electrode and a drain electrode, and the second electrode is the other electrode different from the first electrode. For example, when the first electrode is the source electrode, the second electrode is the drain electrode. The first transistor M1 coupled to the scan line Sn and the data line Dm is turned on and supplies a data signal, which is supplied through the data line Dm, to the storage capacitor Cst. In this operation, the storage capacitor Cst is charged with a voltage corresponding to the data signal.
The gate electrode of the second transistor M2 is coupled to one terminal of the storage capacitor Cst, and the first electrode of the second transistor M2 is coupled to the first power supply ELVDD and the other terminal of the storage capacitor Cst. Further, the second electrode of the second transistor M2 is coupled to the anode electrode of the organic light emitting diode OLED. The second transistor M2 controls the amount of current flowing from the first power supply ELVDD to the second power supply ELVSS through the organic light emitting diode OLED, in response to the voltage value stored in the storage capacitor Cst. In the configuration of FIG. 1, the organic light emitting diode OLED emits light corresponding to the amount of current supplied from the second transistor M2.
However, the pixel 4 of the organic light emitting display device of the related art cannot display an image with uniform luminance. To be more specific, the second transistors M2 (driving transistor) in the pixels 4 may have different threshold voltages for each pixel 4 due to process variation. As the threshold voltages of the driving transistors are different, light with different luminance is generated by the pixels due to the difference in the threshold voltages of the driving transistors, even if data signals corresponding to the same gradation are supplied to the pixels 4.
In order to overcome the problems, a structure including an additional transistor is formed in each pixel 4 to compensate for the threshold voltage of the driving transistor. A structure for compensating for the threshold voltage of a driving transistor using six transistors and one capacitor for each pixel 4 has been disclosed. However, the six transistors in the pixel 4 complicate the pixel 4. In particular, the possibility of malfunction is increased and yield is correspondingly decreased by the increased number of transistors in the pixels.